The present invention is directed to the continuous operation of a plurality of combustion units using a selective catalytic reduction (SCR) system that reduces NOx in the flue gases, and more particularly to a system wherein a spare SCR reactor is installed in parallel with the primary SCR units so that the spare unit can be utilized while any one or more of the primary SCR units are placed out of service for maintenance.
Selective catalytic reduction (SCR) reactors convert nitrogen oxides (NOx), present in flue gases from combustion sources, into a harmless by-product of nitrogen and water. A single SCR reactor is commonly installed for each combustion source for the reduction of NOx. Such combustion sources include but are not limited to refinery heaters, industrial furnaces or boilers.
The performance of an SCR unit is limited by the effectiveness of the catalyst placed within the reactor. Unfortunately, the effectiveness of the catalyst diminishes over time due to catalyst inactivation from sulfur gases and/or other impurities in the exhaust gases. Catalyst suppliers are reluctant to guarantee that a catalyst will last more than 3 or 4 years, even if operating at ideal conditions. Ideal conditions for optimal performance of the catalyst include: 1) operating the catalyst within its specified temperature range; 2) utilizing combustion products with low sulfur compounds such as SO2 and SO3; and 3) no particulate matter present in the exhaust gases. These ideal conditions are difficult, if not impossible, to meet in industrial installations, chemical plants and petroleum refineries. Since these ideal conditions are not met, the life of the catalyst will be cut short and thus require replacement more often than every three or four years. To replace the catalyst, the industrial plants must shut down the facility or exceed desirable NOx emission levels. Either alternative can be costly to the plant in terms of both time and money.
Conventionally, each heater will have its own ducting, SCR unit, fan, emission monitoring system, and stack. Under this arrangement, the probability of catalyst failure to occur within four years is high, thus resulting in an unnecessary shutdown of the plant. Such shutdowns often result in a delay of 2 to 3 days before the plant can resume its normal operations. A single shutdown can burden operations with millions of dollars worth of lost opportunity.
To extend the operational life of the SCR unit and attempt to avoid the unnecessary delays, plant designers have simply added additional catalyst to the SCR unit. The increased amount is usually 25% or more of catalyst to the unit for every extra year of life needed for the unit. This technique is not desirable because the additional catalyst burdens other aspects of the operation, such as: 1) the size of the fan, 2) the size of the SCR reactor, and 3) the additional cost of the catalyst. Adding the additional catalyst to the reactor results in an increase in the pressure drop of the flue gases. In order to maintain a constant pressure of flue gas, the plant operator is required to install a larger fan. Also, a larger reactor is required to handle the increased volume of catalyst. The addition of larger fans and larger reactors generally results in higher capital costs.
Another prior art approach has been to replace all of the catalyst in all of the associated heaters in the section of the plant when one of the heaters has catalyst inactivity. This logic is based on the fact that if one SCR unit has stopped performing, then it is likely the other units are near the end of the useful catalyst life, and the operators do not want to experience another plant shutdown within a few months due to problems with the SCR of a different heater. This replacement technique is inefficient because the catalyst discarded from the functioning SCR units can have substantial remaining viability.
The present spare SCR invention provides a solution that allows industrial plants to continuously operate and replace the spent catalyst in an SCR unit, while simultaneously reducing the need to obtain government waivers for exceeding allowable NOx emissions.
The present invention is directed toward the continuous operation of an industrial selective catalytic reduction (SCR) system. The present invention allows for the replacement of catalyst within an SCR reactor while not disrupting the operations of a heater or other combustion process.
The spare SCR unit is operated by diverting gases from one combustion unit into the spare SCR unit with the use of dampers in the diversion ducts and the insertion of isolation blind plates at appropriate locations. The spare SCR can utilize the fan, the ammonia distribution system, and the emissions monitoring system of the existing combustion unit. The gas is then re-introduced into the same stack. An alternate embodiment eliminates the return ducting and dampers but requires an additional fan, an emissions monitoring system and stack for the spare SCR unit.
Once installed, the spare SCR system can be operated in a variety of ways to solve different problems that may arise in an industrial setting. For instance, when one of the main SCR units loses catalytic activity and NOx emissions rise to an unacceptable level, the spare SCR unit can be operated without shutting down the heater, until the main SCR is serviced for catalyst or fan replacement. Additionally, where one heater in the SCR system is performing below the environmental requirements, or where another heater in the plant is performing poorly, the spare SCR can work in conjunction with the poor-performing SCR thereby meeting the environmental requirements until such time as the heater can be shut down to change the catalyst.
The spare SCR can also remain empty while the need for the unit remains low. This allows plant operators with the means to regulate the amount of catalyst stored as on-site inventory, for example, where there are a number of spare SCR units installed at the plant site, each installed spare serving as a spare for a plurality of on-line SCR units.